OpenTTD is an open source clone of the Microprose game “Transport Tycoon Deluxe”, a popular game originally written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features.
The original game was released in 1995 (see the wikipedia article) and I´ve been playing it now for 15 years. It´s the second oldest game I´m still playing since it´s release (the oldest is Tetris on the original old school Game Boy).
And I really can´t describe what fascinates me on this game. Basically it´s all about building railroad tracks and streets, directing trains and trucks from destination A to destination B. To be fair – plains and ships are also available…OpenTTD changed not much on the original gameplay but did so many improvements. They managed to keep the game not only alive but in a steady development process.
I can´t mention all the features the game has, but most of them are listed here http://www.openttd.org/en/about.
OpenTTD required the original sound, graphics and data files from Transport Tycoon Deluxe. But development has brought things so far that there are open source resources available for free download, making the game TOTALLY FREE.
My advise : check out the game at http://www.openttd.org, download it, install it, play it and become addicted.
Some screenshots from 1.0 (images are very large) :
(Update: removed the pictures, took too long to load, use the links above.)